1. Field of the Invention
The present invention relates to a pressure sensor which can be formed integrally with a flexible circuit board, and which is suitable for the measurement of a pressure distribution used for a flexible curved surface including a spherical curved surface. The invention also relates to a method for manufacture thereof, and a pressure detection module.
2. Description of Related Art
General pressure distribution sensors commercially available in the market are each basically composed of a thin sheet of a quadrangular shape. Due to the sheet shape, such sensors can make measurements on a cylindrical curved surface, but are not suitable for measurements on a flexible curved surface including a spherical curved surface. This is because the sensors are associated with wire-saving technology (matrix wiring) in which signals are collected at pressure points including (N×M) points by the use of (N+M) wires, and so, the shape of a measurement region is limited to a quadrangular shape or the like, due to vertical and horizontal wiring.
In addition, these pressure distribution sensors, which are implemented by PET films and printed circuits, are becoming mainstream in use, but the wiring density is low because of low printing accuracy or migration of the wiring, and the wiring is not flexible and has no or little flexural bending resistance. Moreover, these sensors are not suitable for solder reflow, and hence, it is difficult for component parts to be mounted thereon, involving a problem that each sensor has to be connected to a signal processing device through a large-sized connector.
As a solution to these problems, there has been proposed a technology which achieves a tactile sensor capable of measuring a pressure distribution even on various curved surfaces, and which includes a tree-shaped substrate board provided with belt- or strip-shaped portions, and is based not on matrix wiring but on an IC type wire-saving scheme with component parts such as small-sized ICs capable of performing analog to digital conversion and communications being mounted on a sensor sheet (refer to a first patent document).
In a tree-shaped substrate board of this first patent document, in order to place pressure detection parts and wiring along the belt- or stripe-shaped portions, high-density wiring with a narrow space is required, and it is necessary to directly mount electronic parts on a substrate board for the purpose of IC type wire saving.
For that reason, use of a flexible circuit board used for general electronic equipment, instead of the above-mentioned general method using a PET film and a printed circuit, is suitable for the purpose of pressure detection.
In the first patent document, the construction of pressure sensors applied to the pressure detection parts is assumed to be arbitrary, but if existing pressure sensors are employed, there will be a problem that measurement errors occur, flexibility can not be made high, and reduction in size is difficult.
For example, a pressure detection mechanism used for a general pressure distribution sensor has a mainstream structure in which a pressure-sensitive resistance material or an elastic dielectric material is inserted between two sheets of films (for example, FlexiForce (trade name), Tekscan (trade name), etc., of NITTA Corporation).
However, in such a type of pressure distribution sensor, when it is bent, an internal pressure will be generated, which can not be distinguished from the case where an external pressure acts on the sensor, thus giving rise to a measurement error. This is because the curvatures of individual layers thus bent are different from each other, so in a bent portion, the individual layers try to come near to each other, thereby generating an internal pressure in the normal line direction of the sensor sheet. The value of the internal pressure becomes larger in accordance with the increasing curvatures.
In a bonding process to bond a spacer and a cover film to each other, an adhesive or bonding agent flows into a sensor portion, whereby the conductivity of electrodes is impaired and sensor characteristics are worsened. For this reason, the reduction in size is difficult. In particular, in cases where a plurality of sensors are to be mounted with high density, it is a difficult problem to make effective use of a limited adhesive or bonding area as well as to maintain a hollow structure of an electrode portion.
In a method in which a pressure-sensitive resistance material is coated, an elongation force may be generated along a film surface at the time of bending, thus resulting in an error. At the time of bending, in particular, in cases where the curvature of the bending changes in a dynamic manner, it is very difficult to make a distinction between such an internal pressure and an external pressure which is originally wanted to be measured, and this becomes a great cause of error in the pressure distribution measurement in a flexible curved surface.
In addition, in a sensor using matrix wiring, or in an electrostatic capacitive type sensor in which an elastic dielectric material is sandwiched between films at opposite sides, electrodes and wiring are required to be disposed on the films at opposite sides, so there is a problem that the sensor becomes thick and the flexibility thereof deteriorates. This means that an external force is required for the bending of the sensor itself, which also becomes an error with respect to an external force to be measured originally.
For the pressure detection parts, in addition to such a technique with a pressure-sensitive material, etc., being sandwiched, there is also another technique in which a pressure sensing element is mounted as a component part, but in this case, the pressure sensing element is mounted by means of solder reflow or a conductive adhesive or bonding agent, and hence, durability is low due to the peeling off of solder or the adhesive or bonding agent, resulting from their hardness, bending, etc. Moreover, there is a problem that adhesive bonding is high in implementation cost, etc. In particular, in the case of solder, there is also another problem that an extra or excessive region required in the surrounding of each solder plating portion, due to such as the size of each pad portion, the size of each opening portion of a cover lay, a mounting error, and so on, is large, thus making it difficult to achieve reduction in size and increase in packaging (wiring and mounting) density.
Further, as general pressure-sensitive resistance, there are mainly used ones in which an electrically conductive powder is dispersed in a binder such as rubber, etc., but it can not be said that these have high heat resistance. In order to coexist with solder reflow required for the bonding or adhesion process of the cover films as well as for IC type wire saving, a certain degree of heat resistance is also required, so it becomes necessary to pay careful attention to materials.